单独放置 WHERE 语句以使 UNION 函数起作用

Question

我正在做一个 hackerrank 练习，我必须找到名字最短和最长的城市，如果有很多名字中的字符数量相同，则按字母顺序取第一个

这是提供的table

经过多次尝试，我成功地创建了这段有效的代码

SELECT * FROM (
SELECT CITY, LENGTH(CITY) AS LENGTH_CITY FROM STATION ORDER BY LENGTH_CITY ASC, CITY) WHERE ROWNUM = 1
UNION
SELECT * FROM (
SELECT CITY, LENGTH(CITY) AS LENGTH_CITY FROM STATION ORDER BY LENGTH_CITY DESC, CITY) WHERE ROWNUM = 1;

所以我的问题是，最初我编写的另一段代码不起作用，我想知道为什么我必须将“WHERE ROWNUM = 1”单独放在 SELECT*让 UNION 工作？

SELECT CITY, LENGTH(CITY) AS LENGTH_CITY FROM STATION WHERE ROWNUM = 1 ORDER BY LENGTH_CITY ASC, CITY
UNION
SELECT CITY, LENGTH(CITY) AS LENGTH_CITY FROM STATION WHERE ROWNUM = 1 ORDER BY LENGTH_CITY DESC, CITY;

Answer 1

我会推荐 row_number() 而不是 rownum。使用它，你不需要union，你可以这样做：

select *
from (
    select s.*, 
        row_number() over(order by length(city), city) rn_asc,
        row_number() over(order by length(city) desc, city) rn_desc
    from station s
) s
where 1 in (rn_asc, rn_desc)

如果您使用 union 执行此操作（效率低于上述解决方案），则 row-limiting 子句更合适（从 Oracle 12c 开始可用）：

(
    select * 
    from station 
    order by length(city), city 
    fetch first 1 row only
) union all (
    select * 
    from station 
    order by length(city) desc, city 
    fetch first row only
)

Answer 2

I was wondering why do I have to place the "WHERE ROWNUM = 1" separately in a SELECT* for the UNION to work?

与UNION无关，与ORDER BY有关。

您发布的第二个查询肯定没有按照您的预期执行，因为 WHERE 子句在 ORDER BY 之前执行，因此：

select city
from station
where rownum = 1
order by city

说：给我 table 中的第一个随机行，然后按 city.

对该行进行排序

因此：首先对行进行排序，然后取第一个：

select *
from (select city
      from station
      order by city
     )
where rownum = 1

将其应用于您的情况，长度等等。

单独放置 WHERE 语句以使 UNION 函数起作用

Placing a WHERE statement separately for a UNION function to work

sql

oracle

union

sql-order-by

window-functions